Filliment hum on cathode follower tube question ??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by ChrisGS, Feb 6, 2006.

  1. ChrisGS

    ChrisGS Member

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    Hey everybody ,
    I've been fooling with amps for a little while , but I'm no expert tech .

    I'm sure that I read in some places that because of the higher than typical voltage on the cathode of a tube being used as a "cathode follower" , that the tube in that position can be more prone to filliment hum .

    I had a situation where my friend's amp(Bluesbreaker) started to develope a hum and I changed out that tube (V2) and the hum went away .

    Someone suggested that the reason that the hum went away, must have been because the replacment tube was probably a "weaker" tube and that it was not picking up the hum that's in the amp ...But , I don't think that the amp sounds "weak" .

    His feeling is that TUBES don't hum , AMPS HUM .

    I could swear that I've even had some brand new tubes hum more than others , right out of the box ...and that the quiet tubes didn't seem weaker than the humming tubes ....Am I right , or wrong ?

    Thanks for any replies ,
    Chris .
     
  2. Shea

    Shea Member

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    I think some 12AX7s are simply more prone to heater hum than others, and it has nothing to do with how "strong" or "weak" they are.

    The ones with spiral filaments definitely should have less hum than the ones that don't, by design.

    Also, I wonder if maybe all spiral filaments are not created equal, meaning some brands of tubes with spiral filaments don't do as good a job of eliminating hum as others do. I don't know.

    Shea
     
  3. donnyjaguar

    donnyjaguar Member

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    You found that too, eh? General practice for me when re-tubing is to try each 12AX7 tube in the first amplifier section and use the one with the least amound of noise. DC filaments help a lot here I've found so I incorporated into my last amp build this feature.

    DJ
     
  4. ChrisGS

    ChrisGS Member

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    Hey guys ,
    Thanks for confirming that I'm not nuts , and that some 12ax7 tubes DO hum more than others .

    Can anyone confirm the thing about the " cathode follower" tube having more of a hum problem (because of the voltage on the cathode) ?

    Chris .
     
  5. Fuchsaudio

    Fuchsaudio Member

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    There are a few possible causes: One thing is something called "heater cathode differential". Some tubes don't like too much of a voltage difference between the cathode and heater. Since a cathode follower puts the cathode higher (like 10 to 20 volts, instead of a preamp tube which is like 1 or 2 volts), this can cause hum. Simple solution is raising the filament voltage slightly above ground with a simple circuit (2-resistors and a cap). Adding a hum balance control can help too. DC filaments are also cool too.:RoCkIn

    Second thing is the voltage amp preceeding the cathode follower actually generating the hum, since it's in the same bottle and IT's cathode is NOT high above ground.Don't assume it's the actual follower making the hum.

    The tubes themselves can make a difference. A spiral wound filament (like an ECC-803-S or a 12AX7S) will be quiter, just like twisting filament leads, twisting the filament wires inside the tube matter.

    Finally, it's not always the follower itself (assuming the power supply is quiet and the circuit is properly grounded etc.), as a cathode follower is slightly less than unity gain, so it wouldn't really make much noise of it's own.

    Good luck.
     

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